1. Field of the Invention
The present invention relates to an inkjet head that ejects ink droplets from a nozzle by applying pressure to ink and forms an ink image on a recording medium, and also to an ejection device including the inkjet head.
2. Related Art
As described in Japanese Patent Application-Publication Nos. HEI-1-115638 and SHO-58-119872, there has been known an inkjet head that ejects ink droplets from a nozzle by changing the volume of a pressure chamber using a piezoelectric actuator to apply pressure to the ink.
FIG. 1 shows an example of such inkjet heads. An inkjet head 200 shown in FIG. 1 includes a high-rigidity housing 112, a group of plates 126, and a piezoelectric actuator 114.
A common ink channel 113 and a plurality of openings 112a are formed in the high-rigidity housing 112. An ink introduction pipe 118 is connected to the high-rigidity housing 112 for introducing ink from an ink cartridge (not shown) into the common ink channel 113.
The plates 126 are attached to the high-rigidity housing 112 and include a nozzle plate 102, channel plates 103, and a diaphragm plate 110. A plurality of nozzles 101 is formed in the nozzle plate 102. The channel plate 103 includes a chamber plate 105 and a restrictor plate 107. The chamber plate 105 is formed with pressure chambers 104 arranged in a row, and the restrictor plate 107 is formed with restrictors 106. The restrictors 106 fluidly connect the common ink channel 113 to the pressure chambers 104 and control ink flow to the pressure chambers 104. A diaphragm 108 and a filter section 109 are formed on the diaphragm plate 110. The filter section 109 is formed of a filter plate that has elasticity and removes foreign matter and the like from ink flowing into the restrictors 106 from the common ink channel 113.
The piezoelectric actuator 114 includes a plurality of piezoelectric elements 115 and a securing member 116 that secures the piezoelectric elements 115. Each piezoelectric element 115 corresponds to one of the pressure chambers 104 formed in the chamber plate 105. The piezoelectric elements 115 are housed in the respective openings 112a of the high-rigidity housing 112 and attached to the diaphragm 108. On the securing member 116 are formed individual electrodes 117 for sending independent electrical signals to the respective piezoelectric elements 115 from an external drive circuit (not shown). Applying electrical signals selectively to the piezoelectric elements 115 causes the piezoelectric elements 115 to expand and contract. The diaphragm 108 transfers the displacement (expansion/contraction) of the piezoelectric element 115 to the pressure chambers 104 and changes the volume of the pressure chambers 104. This change of the volume becomes a change of pressure of the ink filling the pressure chambers 104. As a result, ink is ejected through the nozzles 101 as ink droplets.
Usually, the nozzle plate 102 is formed by stainless steel precision pressing, laser processing, nickel electroforming, or the like, and the chamber plate 105, the restrictor plate 107, and the diaphragm plate 110 are formed by stainless-steel material etching or nickel material electroforming. The high-rigidity housing 112 is formed by stainless-steel material cutting or the like.
The processing precision (shape) of the nozzle 101 greatly affects the ink ejection characteristics of the inkjet head 200. In order to suppress variations in position precision of the plurality of nozzles 101, high processing precision is required when the nozzle plate 102 is manufactured.
There is now a continual demand for significantly higher precision of the nozzles 101 in the inkjet head 200. However, if the density of nozzles 101 is further increased, it is difficult from a processing precision standpoint to form the opening 112a for each piezoelectric element 115 in the high-rigidity housing 112. That is to say, high processing precision is required because the expansion/contraction amount of the piezoelectric elements 115 is extremely small (about 0.5 μm), and a slight difference in structure or dimensional values of the opening 112a and the like will fluctuate the amount of deformation of the plates 126, affecting ink-ejection characteristics.
FIG. 2 shows an inkjet head, disclosed in Japanese Patent-Application Publication No. HEI-6-8422, proposed for overcoming the above-described problem. The inkjet head of FIG. 2 includes a chamber plate 206 and a housing 212. The chamber plate 206 is formed with a row of pressure chambers 204. The housing 212 has greater rigidity than the chamber plate 206 and is formed with an opening 212A that extends in the same direction as the row of pressure chambers 204. A plurality of piezoelectric elements 215 is fixed to the chamber plate 206 at positions in the opening 212A that confront the pressure chambers 204. A fixing base 216 formed with a thin-film electrode 219 is attached to each piezoelectric element 215 so that a portion of the thin-film electrode 219 is in intimate contact with the corresponding piezoelectric element 215. A lead 217 is connected to an exposed surface of each thin-film electrode 219. When a voltage is supplied through the lead 217 to the corresponding piezoelectric element 215, the piezoelectric element 215 contracts in its lengthwise direction, that is, the direction indicated by an arrow Z in FIG. 2. When application of voltage is stopped, then the piezoelectric element 215 reverts to its initial state. Because no member is provided in between adjacent piezoelectric elements 215 for guiding the piezoelectric elements 215 in the configuration of FIG. 2, the piezoelectric elements 215 can be aligned in a much higher density than with the configuration of FIG. 1.
If the pressure chambers 204 are formed with a large width to ensure that ink droplets are sufficiently large, then the width of the opening 212A in the housing 212 must also be enlarged. This increases the cross-sectional surface area of the opening 212A. Also, the ejection head must be made longer in the nozzle row direction in order to increase the number of nozzles to increase print speed. This also increases the cross-sectional surface area of the opening 212A.
However, the chamber plate 206 is extremely thin, that is, with a thickness of only about 0.8 mm to 1.0 mm. The section of the chamber plate 206 that is formed with the pressure chambers 204 has a total thickness of only about 0.4 mm to 0.6 mm. Accordingly, if the opening 212A of the housing 212 is too large, then deformation of any one of the piezoelectric elements 215 will deform the entire chamber plate 206 and not just the corresponding pressure chamber 204. The displacement generated by the piezoelectric elements 215 is not effectively used to eject ink droplets. Also, crosstalk can be generated between neighboring nozzles that reduces consistency in speed of ejected ink droplets or otherwise degrades ejection characteristic. Crosstalk can become particularly serious when a great number of piezoelectric elements 215 are driven simultaneously. When neighboring pressure chambers 204 are affected by and deform simultaneously with a pressure chamber 204 that is driven to eject ink, the ink meniscus in nozzles corresponding to the neighboring pressure chambers 204 can vibrate.